Surface roughness is detrimental to precision optical applications. For a theoretical perfect reflective surface, incident light is reflected along a precise angle of reflection. For an actual imperfect reflective surface, the reflected light is scattered in directions other than the angle of reflection.
Scattered light is defined by its magnitude and angular distribution. Optical scatter is characterized through the bidirectional reflection distribution function or BRDF, a function of the ratio of scatter per unit solid angle to the incident power. Machines known as scatterometers, goniometers and gonio-reflectometers are frequently used to map out the hemispheric reflection for determination of BDRF. Present BRDF measurement systems are reasonably sensitive, but that precision suffers at the low end of the measurement range. These measurement systems provide precise and reliable BRDF measurement of surfaces like diffuse white paint, but their measurements get noisy for low scatter materials such as precision mirrors. Current BRDF measurement technologies have difficulty with extremely low scatter samples. While most of the optical energy is reflected in the specular direction, a small amount of energy is scattered elsewhere within the hemisphere above the surface, which contributes to stray light, degrades sensor performance, raises the noise floor and increases with contamination. The measurement of extremely low-scatter optics is complicated and noise limited to about 10−6/sr.
Scatterometers are optical devices used for surface measurements in quality control applications. A typical scatterometer focuses a collimated (laser) beam of light on a surface, and BDRF is determined by measuring the intensity of the reflected light at different angles with one or more fixed or movable detectors.
This invention utilizes a femtosecond (fs) ring laser with counter-rotating short pulses, as discussed hereinafter. Such pulses have a pulse length on the order of micrometers and consist of only a few cycles of visible light. The details of such a ring laser are known in the art and are discussed, for example, in J. Diels et al., Ultrashort Laser Pulse Phenomena, Academic Press, 1996, Chap. 12.2. The ring laser has a laser cavity formed in a continuous path that supports counter-rotating beams. When the ring laser undergoes rotation, one beam experiences a positive Doppler shift and the other beam a negative Doppler shift. Very small differences Δφ in the phase velocity between the two counter-circulating beams result in a frequency difference Δf=Δφ/τRT which can be measured as a beat note between the two output beams. An example of such detection is shown in U.S. Pat. No. 5,251,230 of one of the inventors. Where the two beams interfere, the frequency difference between the beams is detectable as a beat signal. However, at low rotation rates (such as caused by the rotation of the earth), the frequency of the two beams becomes almost identical, and the beams may lock-up because of light scatter from optical surfaces within the cavity. A ‘dead zone’ occurs while the beams are locked, as the beat signal is zero.
It is known that lock-up occurs only when the counter-rotating beams overlap. Using ultra-short pulses in a ring laser gyro minimizes lock-up, as such pulses overlap at only a couple of locations within the cavity. According to J. Diels' U.S. Pat. Nos. 5,363,192 and 5,367,528, the pulses should overlap in a region of minimal scattering to avoid lock-up.
As shown in the '230 patent, the phase difference between the two beams can be controlled by a device that varies the index of refraction driven at the cavity round-trip frequency 1/τRT. In the absence of lock-up between the counter-circulating pulses, the beat note is:                               Δ          ⁢                                           ⁢                      f            ba                          =                                            Δ              ⁢                                                           ⁢              n              *              d                                      λ              *                              τ                RT                                              =                                                    f                ⁢                                                                   ⁢                Δ                ⁢                                                                   ⁢                P                            ⁢                                                                     P                                              eq        .                                   ⁢        1            where Δn is the amplitude of the change of index induced by the modulator of thickness d submitted to the voltage V=V0 cos(2πμ/τRT), and ρ is the optical path length.
Either a moving saturable absorber or a synchronously pumped optical parametric oscillator may be used to fix the crossing point for the intracavity pulse envelopes, and at the same time minimize phase coupling. The absorber is discussed in the aforementioned book, Ultrashort Laser Pulse Phenomena, Chaps. 5 and 12, while the parametric oscillator is discussed in A. Siegman, Lasers, University Science Books, 1986, Chap. 29. The absorber is also discussed in the '528 and '192 patents referenced above.